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    <script>
        // 案例三
        var a=18;
        f1();
        function f1(){
            var b=9;
            console.log(a);
            console.log(b);
            var a='123';
        }
        // 相当于代码如下：
        var a;
        function f1(){
 
            var a;
            b=9;
            console.log(a);
            console.log(b);
            a='123';
        }
        a=18;
        f1();
    </script>
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